In the expanding integrated optical technology, a number of discrete structures have been proposed to serve as optical building blocks for controlling optical signals of specific wavelengths. Such structures include devices with circular, ring, disc, or sphere shaped active or passive cavities, which are useful components for wavelength filtering, routing, switching, lasing, modulation, multiplexing/demultiplexing, and sensing applications.
Loop or ring resonators are well known integrated waveguide components, see for example cascaded ring resonator described in, C. K. Madsen, “General IIR optical filter design for WDM applications using all-pass filters,” IEEE J. Lightwave Technol., Vol. 18, pp. 860–868, 2000.
The basic schematic diagram of this device shown in FIG. 1. An input beam 1 propagates in an input waveguide 2. This waveguide 2 is coupled to a loop resonator 3 by a coupler 4 that has an adjustment (tuning) element 5. An output signal 6 from an output waveguide 7 varies correspondingly to a driving signal 8 applied to the adjustment element 5.
Integration of two couplings to the same loop resonator thus separating an input and output waveguides is also well known in prior art. See for example, FIG. 4 in the U.S. Pat. No. 6,856,641, incorporated herein by references. The schematic basic element of such device is shown in FIG. 2. An input beam 9 propagates along an input waveguide 10. The input waveguide 10 is coupled to a loop resonator 11 by a first coupler 12. A second coupler 13 connects the loop resonator 11 and an output waveguide 14. An output beam 15 depends on coupling ratios of the couplings 12 and 13. Note that the couplers 12 and 13 have the same characteristics in prior art. No separate tuning of the coupling parameters elements was suggested.
Passive or active loops have been realized in material structures such as glass, lithium niobate (LN), polymer-waveguides, doped silica waveguide, optical fibers and others. Most of the proposed micro-ring structures are based on micro-ring waveguide with a large lateral index contrast (air-substrate) such as whispering-gallery-mode (WGM) cavities. Having small diameters and relatively small bending loss, the strongly guiding micro-cavity can be up to orders of magnitude smaller than the weakly guiding waveguide, such as, for example, of LN Ti-indiffused technology.
The major physical design characteristics underlying such performance criteria as FSR, quality-factor, transmission at resonance, and extinction ratio, are the size and material of the cavity, wavelength, the input and output coupling ratio(s) (analogue of a Fabry-Perot etalon reflectivities), as well as the various components of losses, including coupling, scattering from surface irregularities, bending radiation loss, substrate leakage loss or whispering gallery loss, Raleigh scattering, and absorption due to molecular resonances.
The critical gaps, separating the micro-ring cavity from the tangential waveguides or fibers, determine the input and output coupling ratios of the resonator, which, in turn, define the magnitude of the finesse and the at-resonance transmittance. Because of the high optical confinement and short coupling distances, the coupling coefficients are not readily tuned once the device is fabricated, especially in an independent fashion. Tuning of the refractive index typically serves only to adjust the resonant frequencies. In particular, it is difficult to ensure that the two coupling gaps/lengths on two sides of the resonator cavity are matched. The finesse and the extinction ratio of the resonator would be impaired if the coupling factors and resonator phase are not matched for desirable conditions. Such a coupling management problem is relevant also for add/drop coupling in disk/sphere-shaped cavities having proximity-coupled tapered fiber or prisms, see, for example, “Microphotonic modulator for microwave receiver” by D. A. Cohen, M. Hossein-Zadeh, and A. F. J. Levi in Electron. Lett., vol. 37, pp. 300–301, 2001.
In general, while providing the high-density integration potential, small dimensions make efficient adjustments of cavity parameters a challenging task, limiting their direct implication within various optical systems. Alternative methods may require adjustments of intrinsic gain or loss (i.e. rate of energy flow), thus modifying the response shape or use of thermal tuning, see, for example, B. E. Little and Sai T. Chu, “Theory of loss and gain trimming of resonator-type filters”, IEEE Photonics Technol. Lett., Vol. 12, No. 6, pp. 636–638, 2000. The speed and accuracy of thermal tuning, however, are not sufficient for high-speed applications.
Recently, Y-junction reflectors were proposed for effective change in propagation direction in weakly guided planar Ti-diffused LN structures “Tunable lithium niobate waveguide loop” by J. X. Chen at al IEEE Photonics Technol. Lett., Vol. 16, 2090–2092, 2004. In such embodiments, potentially, the add/drop coupling ratios can be readily tuned independently via applied electric fields along with the resonance frequency of the loop. Thus, the speed/accuracy and, as a result, applicability can be much improved over those obtained with thermal or electro-optical tuning with inseparable controls of cavity parameters (coupling and phase), such as, LN-based WGM cavities. Weakly guided technology also may mitigate the additional problem of fiber/waveguide alignment, which should provide an effective coupling between fiber and the micro-cavity and vise versa.
The generic unidirectional coupling between a ring resonator and a waveguide was discussed in the prior art and basic equations describing the manipulation of coupling between optical waveguides and microresonators were obtained for the case of the lossless cavity and/or simplified coupling operator model. For instance, it was shown in A. Yariv, “Universal relations for coupling of optical power between micro-resonators and dielectric waveguides,” Electronics letters, Vol.36, No.4, pp. 321–322, 2000, that full transfer of power from the input port to the output port occurs when the two following conditions are satisfied: 1) all internal losses are negligible, and 2) couplings between the cavity and two waveguides are identical.
Reported methods for adjustable coupling are limited to single coupler tuning by using thermal or electro-optical perturbations of the refractive index, which is inseparable with the cavity phase change. Another reported techniques include additional phase matching structures (e.g. gratings) installed near coupled guides, e.g. waveguides, prisms (half blocks), or (tapered) fibers. Immersing the resonator cavity couplers in liquids may also improve the coupling by reducing the optical fiber modes confinement. Alternatively, the angle of light incidence with respect to the normal of the coupling face could be controlled. U.S. Pat. No. 6,393,186 suggests using “athermal” waveguides and couplers in order to separate thermal phase modulation (considered to be parasitic) and coupling modulation, which is done specifically by thermo-grating elements.
Coupling elements of δβ-reversal coupler configuration are disclosed in, “Directional coupler switches, modulators, and filters using alternating Δβ techniques” by R. Schmidt, R. Alferness in IEEE Trans. on Circuits and Systems, Vol. 26, No. 12, pp, 1099–1108, 1979. The δβ-reversal couplers may have advantages in terms of at-resonance stabilization of the structure since they are not introducing undesired phase shift to the cavity wave.
None of the above described prior art technologies proposed a separate adjustment of coupling ratios of the input and output waveguides coupled to a loop resonator with losses. This fact limits boundaries of loop resonator structures not allowing dynamic, such as real time, optimization of resonator output/performance. In addition, lack of mentioned resonators tunability disables an effective compensation of fabrication errors and/or changes in environmental conditions, thus decreasing the manufacturing yield and productivity.
There is a need for integrated optical devices that allow compensating of losses inherent to loop resonator structures. This compensation must be dynamic to follow changing environmental conditions.